You can now enable automatic application layer DDoS mitigation for Application Load Balancer (ALB) resources, in addition to CloudFront distributions, protected by AWS Shield Advanced. When you protect your resources with AWS Shield Advanced and enable automatic application layer DDoS mitigation, patterns associated with layer 7 DDoS events will be identified and isolated by automatically created AWS WAF rules in your web access control lists (ACLs). These rules can be implemented in count mode to observe how they will impact resource traffic and deployed in block mode at any time. You can also view detection, mitigation, and top contributor metrics associated with application layer DDoS events for further investigation or to assess the effect of any mitigations Shield Advanced creates. These capabilities enable you to quickly respond to and mitigate DDoS events that threaten the availability of your applications.
Source:: Amazon AWS